This invention relates to automatic gain control circuits of the kind including variable gain amplifying means adapted to amplify an input signal, control signal generator means adapted to generate a control signal for controlling the gain of said variable gain amplifying means, and level detection means coupled to an output of said variable gain amplifying means and to an input of said control signal generating means.
A particular application of automatic gain control circuits of the kind specified is in a data modem receiver utilized in a digital data transmission system wherein the data is transmitted in the form of a modulated carrier wave as an analog signal over a transmission medium such as a telephone line. As is well known the imperfect characteristics of the analog transmission medium cause the signal received at the remote receiver to differ from that transmitted, since the signal may be corrupted by various impairments during transmission. In some types of modulation, such as quadrature amplitude modulation (QAM), the amplitude of the received signal is of particular significance. One type of impairment which affects the amplitude of the signal is known as a gain hit and is a sudden change in signal level. Such an impairment may be compensated for by providing an automatic gain control circuit at the input to the modem receiver. Such automatic gain control circuit should have a wide dynamic range in order to provide effective compensation where the unimpaired received signal may vary over a large range, such as about 40 decibels (dB), depending on the losses experienced in the transmission channel. The automatic gain control circuit should provide a reliable signal level prior to any analog-to-digital conversion which may take place in a modem wherein the received signal is processed digitally.
An automatic gain control circuit of the kind specified is known from the article "A review of automatic gain control theory" by D. V. Mercy, published by Institution of Electronic and Radio Engineers, 99 Gower Street, London WC1, England, in The Radio and Electronic Engineer, vol. 51, No. 11/12, December 1981, pages 579-590. See especially FIG. 2 on page 580.
The known automatic gain control circuit has the disadvantage that the incidence of impulse noise in the transmission may adversely affect the performance of the automatic gain control circuit. Thus, the circuit gain may change considerably in response to a noise pulse in view of such a noise pulse typically having a large amplitude. Since the automatic gain control circuit cannot respond to signal level changes instantaneously, the output signal of the gain control circuit in the period shortly after termination of a noise pulse is likely to be at an unsuitable level for further processing whereby errors may arise.